^^^^
Coaches may call it red-shirting but the purpose, in my view, is very different for SAs than for most schools. Generally, red-shirting is done at civilian schools to allow kids to get bigger, stronger, more experienced, etc. without the year counting against their four years of eligibility. It's typically not done to help the athlete prepare academically for the rigors of college b/c, let's face it, most (not all) big-time college football players are not taking a difficult academic schedule.
However, for SAs, if a student is fully qualifiied, including academically, to attend a SA, I think it would be very hard -- if not impossible -- to send that student to NAPs simply to give him another year of football experience. Plus which, I don't believe that the football practices at NAPs and the other SA schools would be equivalent to a red-shirt year -- it's not as if they're regularly playing with the SA varsity team. So, there may be a high percentage of athletes at NAPS, but I think that's b/c there is a high percentage who, for various reasons, need additional academic help.
I think USNA would have a VERY hard time justifying to the Board of Visitors, Congress, and others, that students who are fully qualified to attend USNA are sent to NAPS for a year at taxpayer expense to make them better football players. And I would say the same is true for Army. My guess is the Army coach used "red-shirting" colloquially, but then again, I'm not him and I wasn't there.